There are large reserves of crude oil which cannot or have not been recovered. Most of these reserves are found in geological formations which are considered to be depleted. Of the 442 billion barrels of oil discovered to date at various locations in the United States, less than 30% of that oil has actually been recovered and 70% remains in place. Removal of the 30% of the oil has been accomplished through the use of primary, secondary and tertiary recovery techniques.
Primary oil recovery techniques have depended upon the internal pressure that exists in the oil bearing formation which is called "bottom hole pressure". When a well is drilled from the surface, the internal pressure pushes the oil to the surface or to the well bore so that it can be recovered. After the pressure is exhausted, secondary recovery techniques have been employed. In general, such secondary techniques involve pumping water or sometimes gas into a reservoir so that the oil is swept toward the well where it can be pumped to the surface. Such techniques have boosted a reservoir's average yield to about 25-30% but the remaining 70% has been extremely difficult to release because usually the oil is either too heavy to be driven to the well by the water or gas, or is too diffused, or is trapped by rock and sand.
Removal of some of this remaining 70% can be accomplished through tertiary recovery techniques. Such techniques have not been wholly successful or acceptable, principally due to the cost of conducting the operations. The most advanced of the tertiary recovery techniques involves heating the rock formation sufficiently high to thin the oil and allow it to flow to the well bore. One such method involves the injection of high-temperature, high-pressure steam into the reservoir while an alternative method involves the ignition of the oil bearing rock to push hot gases ahead of the fire thereby causing the oil to thin and flow. With such techniques, oxygen or compressed air must be pumped underground to maintain the combustion process. Although successful, these methods raise the cost of the oil so produced so as to render the technique unacceptable.
Another tertiary recovery technique involves flooding the well with solvents which are miscible with the oil in the formation. Solvents such as kerosene or dry-cleaning agents are pumped into the reservoir so that they mix with and dissolve the oil to clean it from the rock. The high cost of the solvents places one limitation upon the process while another is that the solvents are not recoverable from the rock formations. Carbon dioxide gas has also been injected into the reservoir under pressure. The carbon dioxide dissolves in the oil and reduces the viscosity by a factor of 10 or more thereby causing the oil to flow. While the technique is practical in the laboratory, it has been less so in the field and it also requires the utilization of valuable carbon dioxide gas for the procedure. The most commonly used tertiary recovering techniques involves the injection of a slug of a micellar fluid containing soaps, surfactants, alcohol, oil and water into the rock. The micellar fluid washes some of the oil from the rock and a gel-like thickening agent is then introduced behind the slug to keep the oil moving toward the well. Because surfactants are expensive and because it takes about one year or more to produce oil after the slug has been injected (during which time the well must be sealed to maintain pressure) this technique has not been wholly acceptable either.